Neutronic reactor



April 15, 1958 E. P. wlGNER ET AL. 2,830,944

NEUTRONIC REACTOR Filed Aug. 28, 1945 12 sheets-sheet 1 April 15, 1958 E. P. wlGNER ET AL .y 2,830,944

NEUTROMC REACTOR Filed Aug. 2.8,V 1945 12 sheets-sheet 2 pril 15, 1958 E. P. wlGNER ET AL 2,830,944

NEUTRONIC REACTOR Filed Aug. 28. 1945 f 12 sheets-sheet s vApril 15, 195s E. WIGNER ET AL 2,830,944

NEUTRONIC REACTOR Filed Aug. 28, 1945 12 Sheets-Sheet 4 op o Q o o o o e e o o O o O O v G O O O O O O O O April 15, 1958 E. P. WIGNER ETAL. 2,830,944

NEUTRONIC REACTOR l2 Sheets-Sheet 5 Filed Aug. 28, 1945 enfers:

Y Z'zfyezze MMM April 15, 1958 Filed Aug. 28, 1945 E. P. wlGNER ETAL NEUTRoNIc REACTOR "lauw-s 12 Sheets-Sheet 6 fzzdezzz'ars:

E. P. WIGNER ET AL NEUTRONIC REACTOR April 15, 1958 Filed Aug. 28, 1945 l2 Sheets-Sheet -7 s I W.

E. P. wlGNER ETAL 2,830,944

April 15, `1958A NEUTRONIC REACTOR Filed Aug. 2s, 1945 12 Sheets-Sheet 8 h l@ *l N 12 Sheets-Sheet lO E. P. WIGNER ET AL NEUTRONIC REACTOR April l5, 1958 Filed Aug.Y 28, 1945 NNN April 15, 1958 E. P. wlGNER ET AL 2,830,944

NEUTRONIC REACTOR V Filed Aug. 28, 1945 l2 Sheets-Sheet 11 I April 15, 1958 E. P. wlGNER ET AL 2,830,944

NEUTRONIC REACTOR Filed Aug. 28, 1945 l2 Sheets-Sheet 12 1ljxnitecl StatCS PlliC-I4 Nnurnonuc REACToR Eugene P. Wigner, Alvin M. Weinberg, and Gale J. Young, Chicago, Ill., assignors to the United States of America as represented by the United States Atomic Energy Commission Application August 28, 1945, Serial No. 613,154

4 Claims. (Cl. 204-193.2)

The present invention relates to the subject of neutronics and particularly to a liquid cooled neutron chain reacting system, also referred to as a neutronic reactor, or pile, the latter name having been originally adopted for the active portions of systems employing uranium or other ssionable bodies geometrically arranged in graphite or other moderator in the form of lattice structures. A specic embodiment of the invention has to do with liquid cooled neutronic reactors in which Va coolant passes through tubes of ssionable material. Due to the fact that heat is generated at different rates in different parts of the reactor the flow of coolant through the reactor is regulated in accordance with this variation in heat pro-4 duction.

As a result of the chain reaction, when U23S is present (as in natural uranium), transuranic element 94239, known as plutonium, is produced. This material is lssionable and is valuable when added to natural uranium for use in a chain reacting system, as a ssionable body in lieuv of or in conjunction with natural uranium. Natural uranium contains both uranium isotopesU235 and U238 in the ratio of 1 to 139. The U235 is the isotope iissionable by slow neutrons.

When iission occurs in the U235 isotope, the following reaction takes place:

where A represents flight fission fragments having atomic masses ranging from 83 to 99 inclusive and atomic numbers from 34 to 45 inclusive; for example, Br, Kr, Rb, Sr, Y, Zr, Cb, Mo, Ma, Ru, land Rh; and B represents heavy fission fragments having atomic masses ranging from 127 to 141 inclusive, and atomic numbers from 51 to 60 inclusive; for example, Sb, Te, I, Xe, Cs, Ba, La, Ce, Pr, and Nd.

The elements resulting from the iissions are unstable and radioactive, with half-lives varying in length in accordance with the elementformed.

The absorption of thermal or resonance neutrons by the U238 isotope gives rise to the conversion of U238 to U239 which ultimately decays to transuranic element 94239. The reaction is as follows:

9223B [plus 6 m. e. v. of 'y rays; not necessarily all of one frequency] [600 kv. upper energy limit. Also 2 y rays, 400 kv., and

270 kv., about l of which are converted to electrons] lMost of the neutrons arising from the fission process are set free with the very high energy of above one million .electron volts average and are therefore not in condition to be utilized eiciently to create new thermal neutron iissions in a iissionable body such as U235 when it is mixed with a considerable quantity of U238, particularly as in the case of natural uranium. The energies of the ission-released neutrons are so high that most of' ne ICC o.

created, must be slowed down to thermal energies before there may be a relatively high probability of neutron cap-p Carbon in the form ofl graphite is a relatively inexpensive, practical, and readily as satisfactory as carbon.

pensive, is especially valuable because of its low absorpthey are most eiective to produce fresh fission by reaction with additional U23'5 atoms. If a system can be made in whichneutrons are slowed down without excessive absorption until they reach thermal energies and then mostly enter into uranium rather than into any other element, a self-sustaining nuclear chain reaction can be obtained, even with natural uranium. Light elements, such as deuterium, beryllium, oxygen or carbon, the latter in the form of graphite, can be used as slowing can be obtained, even with natuarl uranium. Light elements mentioned for slowing down fast ssion neutrons is that fewer collisions are required for slowing than is the case with heavier elements, and furthermore, the above enumerated elements have very small neutron capture probabilities, even for thermal neutrons. Hydrogen would be most advantageous were it not for the fact that ture by the hydrogen nucleus.

available agent for slowing fast neutrons to thermal ene'rgies. Recently, beryllium has been made available in sufficiently large quantities for test as to suitability for use las a neutron slowing material in a system of the type to be described. lt has been found to be in every wayY Deuterium while more extion of neutrons and its compounds such as deuterium oxide have been used with very effective results.

However, in order for the premiseto be Vfulfilled that the fast iission neutrons be slowed to thermal energies in a slowing medium without too large an absorption in the U238 isotope of the uranium, certain types of physicaly structure should be utilized for the most efficient reproduction of neutrons, since unless precautions are taken to reduce various neutron losses and thus to conserve neutrons for the chain reaction the rate of neutron reproduction may be lowered and in certain cases lowered to a degree such that a self-sustaining system is not attained.

The ratio of the number of fast neutrons produced by the fissions, to the original number of fast neutrons creatf ing the ssions, in a system of innite size using specific materials is called the reproduction or multiplication factor of the system and is denoted bythe symbol K. If K can lbe made sutliciently greater than unity to create a net gain in neutrons and the system made suiciently large so that `this gain is not entirely lost by leakage from the exterior surface of the system, then a selfsustaining chain reacting system can be built to produce power by nuclear fission of natural uranium. The neutron reproduction ratio, r in a system of finite size differs `from K by the leakage factor and by localized neutron absorberssuch as control rods, and must be sufficiently greater than unity to permit the neutron density to rise exponentially. Such a rise will continue indefinitely `if not controlled at a desired density corresponding to a desired power output.

During the interchange of neutrons in a system comprising bodies of uranium of any size in a slowing medium, .neutrons may be lost in four ways, by absorption in the uranium metal or compound Without producing ssion,

[by absorption in the slowing down material, by absorption in impurities present in the system, and by leakage Patented Apr. 15, 195,8V

Yfists" the 'system'. These lssses'wiu be coia in 'the order mentioned. l

Natural uranium, particularly by reason of its U238 content, has an especially strong absorbing power for neutrons when they have been slowed down to moderate energies. The absorption in uranium at these'energies is termed the uranium resonance absorption or capture. Itis ycaused by the isotope U238 and does not result in fission but creates the isotope U239 which by two successive beta vemissions forms the relatively stable nucleus 94239. It is not to be confusedwith absorption or capture of neutrons by impurities, referred to later. Neutron resonance` absorption in uranium may take place either on the surface of the uranium bodies, in which case the absorption is known as surface resonance absorption, or it may take place further in the interior ofthe uranium body, in which case 4the absorption is known as volume resonance absorption. It will be appreciated that this classification of resonance absorptions is merely a convenientl characterization of observed phenomena, and arises, not because the neutron absorbing power of a U23B nucleus is any greater when the nucleus is at the surface of a body of metallic, or combined uranium, lbut because the absorbing power of U238 nuclei for neutrons of certain particular energies is inherently so high that'fpractically all lneutrons that already happen to have'those energies, .called resonance energies as explained above', are absorbed almost immediately upon their arrival in the body of uranium metal or vuranium compound,and thus in effect are absorbed at the surface of such body.` Volume resonance absorption sdueV to the fact that some neutrons make collisions inside the uranium body and may thus arrive at resonance energies therein.` Aftersuccessfully reaching thermal velocities, about 40 percent of the neutronsare also subject to capture by U238 without fission, to produce U239 and eventually 94239.

i directly available for the chainreaction. Al similar `It is possible, ,by proper physical arrangement of the i materials, to reduce substantially uranium resonance absorption. By the'use of light elements as described above for slowing materials, a relativelyplarge increment of energy lossis achieved in each collision and therefore fewer collisions are required to.v slow the neutrons to thermals energies, thus decreasing the probability ofva neutron being at a resonance energyas it enters a uranium atom. During the slowing process,'however, neutrons are diffusing throughtthe slowing medium over random paths and distances so that the uranium is not only exposed to thermal neutrons but also to neutrons of energies'varying between the emission energy of fission and thermal energy. Neutrons at uranium resonance energies will, if they enter uranium at these energies, be absorbed on the surface of a uranium bodyV whatever its size, giving rise to surface absorption. Any substantial reduction of overall surface of the same amount of uranium relative to the amount of slowing material (i. e. the amount of slowingl medium remaining unchanged) will reducesurface absorption, and any such reduction in surface absorption will release neutrons to enter directly into the chain reaction, i. e., will increase the. number of neutrons available for further slowingl and thus for reaction with U235 to produce fission. li For a given ratio of slowing material to uranium, surface resonance absorption losses of neutrons in the uranium can be reduced` by a large factor from the. losses occurring in a mixture of fine uranium` particles and a slowing medium, if the uranium is aggregated into substantial masses in which the mean radius of the aggregates is at least 0.25 centimeter for natural uranium metal and when the mean spatial radius of the bodies is at least 0.75 centimeter for the oxide of natural' uranium (UQ-g). Proportionate minimums exist for other uranium compounds the exact minimum value being dependent upon the uranium content and the density of the product. An important gain is thus made in the number of neutrons ill) o through.

gain is madefwhen the uranium has more than the natural content of ssionable material. Where a maximum K factor is to be desired we place the uranium in the system in the form of spaced uranium masses or bodies of substantial size, preferably either of metal, oxide, carbide, or other compound or combinations thereof. The uranium bodies can be in theform of layers, rods or cylinders, cubes or spheres, or approximate shapes, dispersed throughout the graphite, preferably in some geometric pattern. The term geometric is used to mean any pattern or arrangement wherein the uranium bodies are distributed in the graphite or other moderator with at least either a roughlyuniforrn spacing or with a roughly systematic non-uniform spacing, and are at least roughly uniformin size and shape or are systematic in variations of size or shape to produce a volume pattern conforming to a roughly symmetrical system. If the pattern is a repeating or rather exactly regular one, a system embodying it may be conveniently described as a lattice structure. Optimum conditions are obtained with natural uranium by using a lattice of metal spheres.

The number of neutrons made directly available to the chain reaction by aggregating the uranium into separate bodies spaced through the slowing medium is a critical factor in obtaining a self-sustaining chain reaction uti lizing natural uranium and graphite. The K factor of a mixture of fine uranium particles in graphite, assuming both of them to be theoretically pure, would only be about .785. Actual K factors as high as 1.07 have been obtained using aggregation of natural uranium in the best known geometry, and with as pure materials as it is presently possible to obtain.

Assuming theoretically pure carbon and theoretically pure natural uranium metal, both of the highest obtainable'densities, the maximum possible K factor theoretically obtainable is about 1.1 when the uranium is aggrev, gated with optimum geometry. Still higher K factors can be obtained by the use of aggregation in the case of uranium having more than the naturally occurring content of fissionable elements. Adding such issionable material is termed enrichment of the uranium.

lt is thus clearly apparent that the aggregation of the uranium into masses separated in the slowing material is one of the most important, if not the most important factor entering into the successful construction of a selfsustaining chain reacting system utilizing relatively pure natural uranium in a slowing material such as graphite in the best geometry at present known, and is also important inl obtaining high K factors when enrichment of the uranium is used.

Somewhat higher KA factors vare obtainable where moderators such as deuterium oxide or beryllium are used. Thus with beryllium it is possible to secure a K factor as high as 1.10 with optimum geometry and absolute purity. Moreover with deuterium oxide K factors of about -l.27 may be obtained. When such moderators are used the problem of aggregation may be somewhat less important although it is an essential factor if maximum K factors and minimum size reactors are to be obtained. l l v The thermal neutrons are also subject to capture by the slowing material. While carbon and beryllium have very small capture cross sections for thermal neutrons, and deuterium still smaller, an appreciable fraction of thermal neutrons (about l0 percent of the neutrons present in the system under best conditions with graphite) is lost by capture in the slowing material during diffusion there- It is therefore desirable to have the neutrons reaching thermal energy promptly enter uranium.

' In addition to the above-mentioned losses, which arc inherently a part of the nuclear chain reaction process, impurities present in both the slowing material and the uranium add a very important neutron loss factor in the chain. The effectiveness of various elements as neutron absorbers varies tre1'nend`ously.Certain4 elements such far as possible all impurities capturingneutronsA to the l detriment of` the chain vreaction, from both the slowing material and the uranium. If`these impurities,` solid,

liquid, or gaseous, andin elemental or combined form,

are present in too great quantity, in the uranium bodies or the slowing materiali or in,`or by` absorption frorn,the free spaces of the system, the self-sustaining chain reaction cannot be attained. v The amounts of impurities that may bepermitted in asystem, Vary with a number 'of factors, such as the specific geometry of the system, and the form in which the uranium is used-'L -that is, whether natural or enriched, whether `as metal Aor oxide--andl also factorssuch as the weight ratios between the uranium and the slowing down material, and the, type of slowingdown or moderating material used-a-for example, whether deuterium, graphite or beryllium. Although all of these considerations influence the actual permissible amount of each a impurity material, it has fortunately been `found that, in

general, the elfect of' any given impurity or impuritiescan be correlated directly with the weight of the impurity present and with the K factor of the system, so that knowing the K factor for a given geometry and composition, the permissible amounts of `particular impurities can be readily computed Withouttakinggindividual accountof the specific considerations namedabove. Different impurities are found to alect the operation to widely dilerent extents; for example, relatively considerable quantities of elements such as hydrogen may be present, and, .asV previously suggested, the uranium may be in the form of oxide, such as U02 or U3O8, Vor carbide, although the'v lmetal is preferred. VVNitrogen may be present to `some extent, and its effect on the chain reaction is such that the neutron reproduction ratio of the system may be changed by changes in atmospheric pressure. This effect may be eliminated by enclosing orevacuating Vthe system' if de as danger coefficients which are assigned to thevarit ous elements. These danger coeicients for the impurities are each multiplied by thepercent by weight of the corresponding impurity, and the total sum of these products gives a value known as the total danger sum. This total t danger sum is subtracted from the reproduction factor K as` calculated for pure materials and for the specific geometry vunder consideration. p ,a The danger coeicients are dened in terms of the ratio of the weight of impurity per unit mass of uranium and are based on the cross section for absorption `of thermal neutrons of the Various elements. These values may be obtained from physics textbooks on the subject and` the danger coeflcient computed by` the formula di Au du A;

wherein u, represents the cross section for the impurity and au the cross section for the uranium, A, the atomic weight of the impurity and Au the atomic Weight for uranium. lf the impurities are in the carbon, they are computed as their percent of the weight of the uranium of thesystem.

Presently known values for danger coeicients for some elements are given in the following table, wherein the lements are assumed to haveA their naturaliso'topic con` Y Elementi, Danger'` l 4 g j 'Coeclenh l He 0 Li 310 B 2,150 N 4.0 F 0.02 Na 0.65 Mg 0.48 A1; 0.30 Si `0.26 P 0 3 S-- 0. Cl.- K 2.1 Ca '0.37, Ti.- 3.8 V A 4 Cr 2 Mn--. 7.6 FP 1:6 C0 17 Nl `3 Cn 1.8 0.61 1 2 6.3 2.5 50 t 18 870 54.2 0.18. 1.6 1.6 0.30 1, 430 435 6, 320 0.03 Bl 0:0025 Th 1.1

,sari

stitutionunless' otherwiseindiatedgand areY Where an element is necessarily used in an activepart of `a system, it is-still to be considered as animpurity';

for example,.ina structure where `the uranium bodies con,- sist of uranium oxide, the actual factor K would ordinarily `be computed by taking that` fact into account using as a base K a valuecomputed for theoretically pure uranium.

`As a specific example, if the Ymaterials of the system under consideration have .0001 part by Jweightj of C and Ag, the totaldanger sum in K units 'for such an analysis would be: '7 'l This would be a rather unimportant reductionin the` reproduction factor K unless the. reproduction factor for a given system,` without considering any impurities, is very nearly unity. If, on the other' hand, the impurities `in -the uranium in the previous example hadbeen Li, Co,`and Rh, the'total danger sum would be:

. .031o+.o017+.o050=.0377 K units This latter reduction in the reproduction factor for a given system would be serious and `might well reduce the reproduction factor below unity for certain geometries and certaintmoderator's so as to make it impossible to effect a self-sustaining chain reaction with natural uranium and. graphite, but might,` still be permissible when using enriched uranium in a system having a high K factor. a

This strong absorbing action'of some elements renders a self-sustaining chain reacting system capable of con-- trol. By introducing neutron absorbing elements in the form of rods or sheets into the interior of the system,

for instance'in the slowing material between the uranium masses, the neutron reproduction ratio of the system can be changed in accordance with the amount of" absorbing material exposed to the neutrons in the system. A sufficient mass of the absorbing material can readily be inserted into the system to reduce the reproduction ratio ofthelsystem to less than unity and thus stop the 'reac-z tion. Consequently, it is another object of our invenv large.

'tion Ato provide* a means admethod of lcontrolling hii-ectibnfin a :self-sustaining system; n

When the uranium and"the slowing material are of the such purity and the uranium is so aggregatedthat fewer tof neutrons .present willincrease exponentially and indefinitely, provided the structure is made sufficiently I If, lonthel contrary,.the structure -is small, with a large surface-tovolume ratio,"the're will be `afrate fof loss of neutrons vfrom the structure by leakage through lljie outer surfaces, 'which may overbalance'the rate of i'ieutron production inside Vthe'structure lso that a chain reaction will not be 'self-sustaining. For each 'value ofv the reproduction factor K greater than unity, there is thus a minimumoverall Ysize of a given structure known as the critical size, above which the rate of lossrof'neutr'ns by diffusion Vto the walls of the structure and leakage away from the structure is less than the rate of proI duction of neutrons Within'the. system, thus making Athe *chain reaction-self-sustaining. The rate `.of diiusion of neutrons away from a large structure in which they are being created through Vthe exterior surface thereof may be treated by mathematical analysis when the value of K and certain other constants aregknown, as the ratio of the exterior surface to thc volume becomes less-as the structure is enlarged. v

'In 'the 'case 'offa spherical 'structurelemploying uranium b'odies'imbedded in graphite in `the geometries disclosed herein `and without an vexternal rellector the yfollowing formula gives the critical overall radius (R) in feet: l

where. Gis a-constantthat varies-slightly Vwith geometry of thelattice .andA for normal graphite lattices may have a value close to 7.2.

For a rectangular parallelepiped structure rather than spherical, lthe critical size can be computed from the formula t where tz, and c are the lengths of the 'sidesA infeet. The critical :size .for a cylindrical structure is given by the formula, irrespective of the shape of the uranium bodies y cyiinaertheighr it, ft. Radius R, ft.

'newer/er, when einen Size :is attained, by definition noe'ris'e'in'nutron density'canbe expected. lIt is thereforenecessaryto increase the size of the structure beyoud ltheci'iticl size .but not `to the extent Vthat the pe riodfor do'bliug'ofytlie neutron density :is too short, as Will be explained' later.l Reactors having a reproduction ratio (r) for an operatingstructure with -all control absorbers-removed and .-at' the temperature of operation up toffabout `1:005 arevery easy to controll Reproduction ,ratio-should not be permitted to rise above about 1-.01 lsince the reaction'will become difficult 'to control. Theisize? atV Which'thi's reproduction ratio 'can be obtained maytbe' lcomputed #from modifications of fthe-above 'forasada@ fthe uraniumVV willvsupporty a chain reaction pro- 1f' the 'y `B mulae for critical size. For` example, for spherical active structures the -formula C'Y .`K- T= may ,be lused to ind R when K is known and r is somewhat-over unity. 'Il'he same formula will, of course, give rV forgiven structures for which K and R are known. Critical size may be attained with a somewhat smaller structure by utilizing a neutron Vreiiecting medium surrounding the surface of the active structure. For example, a. 2 foot thickness of graphite having low impurity content,v completely surrounding a spherical structure is effective in reducing the diameter of the uranium bearing portion by almost 2 feet, resulting in a considerable saving of uranium or uranium compound. The'rat'e of production of element 94229 will depend outhera'te of neutron absorption'by U238 and isY also proportional to the rate at which issions occur inl'J235 Ihisin turu'is controlled by the thermal neutron density existing in the reactor while operating. Thus for maximum production of element 94239, it is essential that the thermal neutron density be at a-maximum value commensurate with thermal equilibrium.

.Considerable heat is generated during a neutronic reaction primarily as the result of the fission process. Following are tables showing more specically the type of heat generated in the reactor.

. Summary by type M. e. v./ Percent fission Gamma radiation .L 18 Beta radiation 16 8 Kinetic energy of ssion fragments 160 80 Kinetic energy of neutrons 6 3 Summary by locale where heat is generated M. e. v./ Percent ssion In uranium. 174 87 In moderator 16 8 Outside pile. 10 5 Summary by type and locale M. e. v. Percent Percent Percent per in U in C Outside fission Kinetic energy of fission fragments 159 100 Kinetic energy of neutrons... 6 99 l Gamma radiation from nssion products 5 50 45 5 Beta radiation'from fission products .v 6 100 Nuclear anity of neutrons (gamma radiation) 12 70 25 5 When the system is operated for an extended period of time at a high production output of element 94239, the large amount of heat thus generated must be removed in order to stabilize the chain reaction. Most of the heat in an operating device is generated as the result of the nuclearfssions taking place in the U235 isotope. Thus,`

the rate of heat generation is largely proportional to the rate at which the iissions take place. Inother words, if the rate of generation of neutrons is increased, a greater amount of coolant must be passed through the reactor in order to remove the heat thus generated to avoid damage, particularly at the central portion of the pile, hyexces-I sive heat. Thus, the highest obtainable neutron density at which a system can be operated for an extended period of time is limited by the rate at which the generated heat c an be'removed. That is to say, the maximum power output of a system is limited by the capacity of the cooling system. An effective cooling system is therefore a primary requirem-entfor high power operation of a neutronic reactor and it has been found that this cooling may be accomplished most effectively by passage of the coolant in contact with or in close proximity to the uranium.

After the neutronic system has operated` for a period of time sufficient to cause a quantity of element 94299 to be produced, it may be desirable to remove at least some of the uranium rods from the reactor in order to extract `element 94299 and the radioactive fission products, both being formed in the uranium rods or for other purposes. In many neutronic reactors, a neutron density variation occurs across the reactor; that is, the neutron concentration at the periphery is relatively small and increases to a maximum value at the center. Actually, therefore, since the rate of production of element 94239 is dependent upon Vthe'neutron density, the reactor will have zones which may be likened to three dimensional shells, the average concentration of element 94299 being uniform throughout any .given zone. In a reactor built in the form of a sphere these would, of course, be in the shape of concentric spheres of different diameters, while one built in the shape of a cylinder would have similar zones but of different shapes.

Where this variation in concentration exists in a reactor it is often desirable to resort to a systematic schedule' of removal depending upon the time of operation and the location of the uranium for removing and discharging uranium metal that has been subjected to neutron bombardment. In the case of a new system of this character the operation would normally continue until the metal in the center portion of the reactor reaches a desired content of element 94299, at which time this metal would be removed and replaced with fresh metal. The next removal then would be from the section next adjacent to the center section of the reactor where the desired content of element 94299 is reachedafter further operation. The process would then proceed with the removal of the metal at various times until the metal recharged at the center of the reactor has reached the desired content of element 94299. This would then be replaced and the t process of progressing towards the periphery continued wtih periodic return to more central areas. Since the neutron density in the central areas of such a reactor would, ordinarily, greatly` exceed the neutron' density near the periphery, the metal in the central areas may be replaced several times for each replacement of the metal near the periphery. A removal schedule can be developed by ycalculation and checked by actual experience after the system has been placed in operation. Y

Different schedules may be developed with other reactors having diferent reactivity curves. For example, certain reactors `are constructed in a manner such that the neutron concentration is substantially uniform throughout a large volume of the reactor. In such a case the schedule for removal of uranium bodies may be modified accordingly. p

Since the heat generated in the reactor results from fissions in the uranium, it is evident that this heat is not formed uniformly throughout the reactor but that it must vary across the reactor with the local rate at which fissions occur and element 94299 formed. Consequently, the relative values for the production of element 94239 applyA also to heat distribution; that is, the heat generated may increase from a minimum atthe outer surface of the reactor to a maximum at the center in certain reactors.

As the total weight of the radioactive ssion elements is proportional to that of the 94299 at the time of fissions, it might be assumed that theV amounts ofthese radioactive `fission elements and of 94299 present in metalv rea-,sadecc moved from the reactor are also of the same proportion. This is ,not true, however, as the fission elements when produced are `highly radioactive and immediately start to decay, some with short half-lives and others with longer half-lives until, through loss of energy, these unstable fission elements arrive at a stable non-radioactive element or isotope and no longer change. The 94299 on the other hand is a relatively stable element when formed, having a radioactive half-life of about 2 l04 years.

At the start of the reaction in new metal the radioactive fission elements and the 94299 both increase in amounts. After a certain period of operation during which time the metal is subjected to intense neutron bombardment the radioactive fission elements will reach a i state of equilibrium and from that time on the amounts of these radioactive elements remain constant, as the fission elements with shorter half lives are reaching a stable condition at the same time new ones are being produced. The amount of the stable end products of fission, however, continues to increase with the increase in element 94239. Consequently, the rate of formation of the fission end products is dependent upon the location of any particular metal in the reactor, and the power at which the system operates controls the maximum radioactive fission element content regardless of the length of time the system operates after equilibrium occurs. The quantity of element 94299 on the other hand, and of the final and stable end products of ssion continue to increase as the operation of the system continues. The amounts of both 94299 and fission end products present are controlled only by the location of the metal in the reactor and the time and power of operation. The highly radioactive ssion elements may, therefore, vary from a substatial percentage of the weight ofelement 94299 present in the metal at the center of the reactor after a short period of operation, to a very small percentage in metal from a position near `the periphery of the reactor after an extended operating period at a given power.

It is not to be assumed, however, that the fact` that equilibrium can be obtained Vbetween the original highly radioactive fission elements and the stable fission end products that all radioactivity will cease when the original fission elements have been permitted to decay for a time equal to the equilibrium period, for example. Many of the original fission elements have long half lives that, taken together with their successive radioactive disintegration products existing long after thel ,fission elements having a shorter half life have decayed, renders the uranium still radioactive especially after prolonged bombardment at high neutron densities. In addition, the successive radioactive disintegration products of the original shorter lived fission elements may still be present.

The equilibrium radioactivity is so intense that metal taken from the reactor for the recovery of element 94299 and fission products immediately after bombardment at high neutron densities will heat spontaneously due to selfabsorption of the intense radioactivity of the remaining.

radioactive fission products. The amount of heat generated as the result of the spontaneous heating will depend particularly on three factors: (l) the concentration of element 94299 and fission products in the metal; (2) th period of time for continuous operation required to reach this concentration; and (3) the elapsed time since the reactor was shut down and the metal was removed.

The metal from the center of the reactor in a systeml operating at a high power output, for example, at a 94299 concentration of l to 2,000, if not cooled, can increase in temperature at the rate of about 2000 C. per hour one day after the neutron activity of the system has been shut down. After 30 days shut down following an operation of days at an output of 500,000 kilowatts, the average temperature rise can be approximately 572 C. per hour.; The uranium metal of the type used in the chain reactingY '711 systems herein under consideration melts at about 1lQO C. i l v v i Under these conditions` uranium bombarded with neutrous for an extended periody of time at high rates of power output'can be safely removed from the reactor under one of the following methods:

(l) The neutron activity of the system is shut down and the uranium is kept in the reactor and continuously cooled until the radioactivity decays to a point where the metal can be removed without melting in ambient air. This procedure may require that the metal remain iu the reactor for a period of from 30 to 50 days after the neutron bombardment has ceased.

(2) The neutron activity of the system is shut down and the uranium is kept in the reactor with the cooling system in operation for only a few days to permit the morst violent radioactivity to subside and then the metal is removed from the reactor with the cooling discontinued during the removal except for cooling by the atmosphere or by water spray. The metal is then promptly placed under more eiiicient cooling conditions before the teinperature of the uranium has become excessive.

(3) The neutron activity of the system is shut down and the uranium is removed while cooling the uranium body at least to an extent sufficient to prevent the temperature from becoming excessive. y

Methods l and 2 above can he accomplished with the reactor herein to be described, while method 3 cannot be accomplished with the reactor shown.

It is also important, of course, from the point of-view of biological safety of operating personnel that adequate shielding be provided to absorb the strong gamma radiations from the fission products present in the active uranium while being removed from the reactor. The neutron activity in the reactor completely ceases within 30 minutes after shut down of the neutronic reaction during which period delayed neutrons are being emitted. In no case then should the uranium be removed from the reactor immediately following shut down of the neutronic reaction, but suliicient time should be given to permit all delayed neutrons to be emitted. Thus, the shielding required during the removal of the uranium rods from the system is primarily intended to protect the personnel from gamma radiations. As stated above, immediately following shut down of the neutronic reaction, there are many short lived radioactive fission elements in the uranium causing the gamma radiation to be very intense. Many of these elements decay into more stable products within the lirst thirty minutes following shut down of the reaction. Thus, the fission products lose a large amount of `their radio activity during this period.

While the method of extracting the fission products and element 94239 from the bombarded uranium'taken from the reactor forms no part of the present invention, the fission products and element 94239 are removable and when removed are extremely useful. The radioactive iission products are valuable for use asiadiation sources, many having long half lives with high energy gamma radiation sufiicient for radiography of even heavy metal castings. In addition, some of the fission products are useful as radioactive tracers in biological and physiological research.

Element 94239 is exceptionally useful because it is vssionable by slow neutrons in thesame'manner as the uranium isotope 92235 contained in natural uranium. The separation of 92235 from 92238 in natural uranium is extremely ditlicult since both are isotopes of the same element and these isotopes vary only a small percentage in comparative weight. Element 94239 on the other hand,

is a different element from uranium, having different chemical properties than uranium, and therefore can be chemically separated from uranium. After separation, for example, element 94239 can be added to natural uranium to supplement the 92235 content, thus increasing the amount of fissionable material in the uranium. This enriched uranium can then be used in neutronic systems making it possible to provide more cooling facilities, for example, than can be used in a system of the same geometry employing only natural uranium. Thus, au enriched neutronic system may provide a greater power output than would be possible in a' natural uranium system having the sameA geometry.

To summarize, the present invention is concerned with a liquid cooled neutronic reactor capable of generating large quantities of heat and of producing element 94239 and radioactive vfission products and is well adapted for long and continuous operation. The coolant is selectively apportioned throughout the reactor so as to remove the heat in accordance with its rate of generation.

The foregoing constitute some of the principal objects and advantages Yof the present invention, others of which will become apparent from the following description and the drawings, in which: v Y

Fig. `l is a schematic drawingof the complete power plant and associated cooling system;

Fig. 2 `is .a schematic drawing of the reactor shown disposed belw the level of the ground;

3'is an enlarged, vertical sectional View taken through the reactor shown in Fig. 2 and illustrating the arrangement of the uranium and graphite and the dispos'ition'of the water and tubes forming the cooling system;

Fig. 4 is a horizontal sectional view taken through the walter tank atthe top of the reactor showing the arrangementof the tank segments and the tubes extending downwardly-from the upper tanks;

Fig. ,5 yie an enlarged, fragmentary, horizontal sectional view taken. on the line 5 5 of Fig. 3;

Fig. 6l an enlarged, fragmentary, detailed sectional view taken vertically through the reactor showing one uranium rod lor tube and its relationship to the graphite and other elements making up the reactor;

Fig.l Tis yan* enlarged, fragmentary, vertical sectional view takenat the bottom ofthe uranium rod and showing thewater vapor seal; i

Fig. 8 is an enlarged, horizontal sectional view taken through one of the uranium rods showing the aluminum tube at center;V

Fig. A9.(is an enlarged, fragmentary, vertical sectional view throughthe bottomportion of a uranium rod assembly showing amodiedV form of water vapor seal at the bottoni; u y k i 1 vis an enlarged, fragmentary sectional view takenhon lthe line 1li-l0 of Fig. 4, the uranium rods beingomitted for purposes of clarity.

'Figl *1'1" isi an enlarged detailed view of one of the control'vs'rodss'hown in Fig. 10;. i

vl`ig.` l 2 `i.: a -horizontal section view taken en line 12-`12 of Figli;

lFig. 13, is an enlarged, horizontal sectional view taken online 'I3- 13' of Fig. l0;

Figi 14 a diagrammatic view showing the control systeinuforlthe pile, the electrical circuit being reduced to the lowest terms; I

*,Fig. l5 a vertical sectional view, partly inl elevation, of av second' lembodiment of the invention;

Fig. `16vrlis avertical sectional view (partly in elevation),' oiflthe reactor shown in Fig. l5 and taken on the line l6'-U16 of Fig. l5;`and

Fig. 1V7'is a fragmentary perspective View partially brokenaway showing the arrangement of uranium slugs inthe aluminum tubes.

Referring to Fig. l the reactor is generally indicated at r20. It includes a pile of bodies containing uranium geometrically s'pacedin graphite blocks. The. specific details of the pile will be explained presently. The heat generated infthe pile as the result of the chain'reactioniisl removed from the reactor by means of waterin a cooling system which may be divided into two circuits;4

. 13 viz., aprimary cooling circuit indicated at A and a secondary cooling circuit indicated at B in heat exchange relationship with the primary circuit.

The primary cooling circuitA comprises a pump 23 circulating waterl throughan ,inlet pipe 24 into the reactor partly through Va reducing valve 115 and medium pressure pipe 24a into a ring headerv ZSyat the top of the reactor 20, and partly through a high pressure pipe 55 into a header 55a (Fig. 4).` `The coolant entersthe reactor 20 and passesvertically down through the pile, as shown in Figs. lv and 3 and, as will be explained later, leaves the reactor through outlet pipe 26, passing through a .water trapzato prevent gas escape from the outlet water tank, k an outlet sump 26h, then through a heat exchanger 27, and finally through a pipe 28 to the pump 23 for. recirculation, 'I hus the outlet water of the reactor is controlled by gravity alone.

The secondary cooling circuit `B serves to cool the water in the primary circuit and includes a pump 29 discharging into a pipe 30, which empties into the, heat exchanger 27.` The cooling medium leaves the heat exchanger .27 through a pipe 31 and passes to a spray head 32 in a cooling tower 33 vwherein the `water spray is subjected to currents of air produced by a suitable blower fan 34. By means of evaporation of a certain percentage of the water `in `the cooling tower 33, the remaining water is cooled and collects in a suitable pan or, sump 35 atthebottom of theV cooling tower, from which the pump 29 draws the coolant through apipe 36. For purposes of illustration, water leaving the reactor 2t) may be at a temperature of approximately 200 degrees Fahrenheit, and `is cooled in the heatv exchanger 27 to a temperature of about 85 degrees. Fahrenheit. These conditions illustrate a suitable temperature dilerential between the cooling water' entering and leaving the reactor 20. water leaving the heat exchanger 27 through a pipe 31 is at a temperature of approximately 130 degrees Fahrenheit, and this liquid is cooled by a process of evaporation in the cooling tower 33 to a temperature of about v75 degrees Fahrenheit. Thus, the temperature diierential between the water entering and leaving the heat exchanger 27 is about 55 degrees.

InV the secondary cooling circuit B, the,

Because intense gamma radiation and vboth fast and slow neutrons escape from `the pile, it is essential to surthe level of the ground indicated at 37. The bottom and sides of the reactor are surrounded by earth 3S so that escaping neutrons and harmful gamma rays are absorbed in the earth, and by this means protection is provided to individuals working about the reactor. Further shielding is required across the top of the chain reacting unit, details of whichwill be describedpresently.

The vchain reacting pile itself may take a variety of shapes and arrangements. Uranium metal and carbide are preferredto furnish the neutrons for the chain reaction, though other uranium compounds may be used, as, for example, uranium oxide. Also, combinations of these materials may be employed.

Referring to Fig.`3, a specic form of the invention is illustrated, wherein the pile takes the form of a cylinder about 51/2 meters high` and V8 to 9 meters in diameter. Uranium metal and uranium carbide `are used in the form of tubes, each having an outside diameter of about 3.6 centimeters, an inside diameter of about one centimeter, and a height of about 51/2 meters. These tubes are arranged approximately 20 centimeters apart, and in all there are about 1300 tubes. This is for a system ernploying in the axial portion about 9 metric tons of uranium metal tubes of high purity, with a peripheral portion of about 63 tons of uranium carbidetubes of high purity, the tubes being geometrically arranged in approximately 480.tons of high quality graphite in the form of rectangular blocks.

If` uranium metal is used throughout the reactor, then' the overalldiameter of the cylinder would be about 7.14 meters, the height about 4.5 meters, the overall volume about V cubic meters, and the Weight of urnaium metal approximately 42 tons.

A sphericalshape for the reator would be somewhat more efficient, and' consequentlythe overall size would be smaller. As a specific example, an operative water cooled pile of spherical shape would be 6.48 meters in diameter, and would have a volume of about 142 cubic meters. The amount of uranium metal in such a structure would be about 33 tons.

If heavy (as opposed to ordinary) water is used as the moderator instead of graphite, the overall size of the reactor would be still smaller.

The reactor rests on a concrete door and is surrounded' by a concrete wall 39, which completely Asurrounds the reactor unit on its sides as shown in Fig. 3. The chain reacting pile in the reactor 20 consists of the vertically disposed uranium rods or tubes 40 surrounded by the moderator, such, for example, as graphite 41.

Referring to Fig. 5, the uranium tubes 40 are shown spaced in a hexagonal arrangement in the graphite 41. It is recognized that other spacing arrangements are equally suitable-such, for example, as a square disposition of the tubes.

Again referring to Fig. 3, the rods 40 are disposed vertically and extend froma position substantially at thev top of the concrete wall 39 to a heavy iron floor 42 spaced above the bottom of the foundation. Between the door 42 and the bottom of the foundation is a discharge tank or header 43 extending over the entire floor area of the foundation and having a depth of about V2 meter. This tank serves as the outlet or discharge header `for the water in the cooling `circuit Vfrom which the water runs by gravity, thus preventing complete lling. The floor 42 may be supported in any suitable manner, vsuch,'for example, as by posts 44 resting on the bottom of the discharge header 43.

As is apparent in Figs. 3 and 5, a space 45 is provided between each uranium rod 40 and the surrounding graph-v ite 41 to provide room for expansion of the uranium rodsV caused by the high temperatures resulting from the neutron chain reaction, and to reduce the amount of fission fragments reaching the graphite from lissions occurring at the surface of the rod.

Referring to Fig. 5, each of the uranium rods 40 is provided with a continuous central vertical passage 46 throughout its entire length. A hollow tube 47 extends through this passage and provides protection as a liner for the inside surfaces of each uranium rod 40. The purpose of this tube is to keep the water from direct contact with the uranium while the water is flowing through each rod, and the tube may be made of aluminum, beryllium, magnesium, tin, lead, or bismuth, for example. Aluminum orV beryllium are preferred. The important feature of thesek materials is that they prevent corrosion of the uranium tubes due to the reaction of water on the uranium, which is particularly serious when the water is at temperatures close to the boilingpoint, and also when this coolant is exposed to the neutron bombardment.

As shown, each of the tubes 47 is made of aluminum and each has an inside diameter of 1 centimeter. The thickness of the tube Wall is about 1 millimeter. The water tubes 47 extend downwardly through suitable open` ings in floor 42 into the discharge tank 43.

Cooling water, passing internally through uranium tubes as shown herein, reduces the value for the reproductionratio for the pile below what it would be without the presence of the Water and cooling ducts, by an amount equal to about l percent for each percent of water byv weight in the pile to the weight of the uranium in the pile.

The thickness of the water layer when passing throughv the reactor unit then must be so proportioned that the' reproduction ratio is maintained at least above unity, forf otherwise the chain reaction would not be self-sustaining,y

d ue t excessive neutron capture by the water.

Since the purpose of the water passing through the tubes 47 is tocool the uranium tubes 40, it is important that the aluminum or other suitable tubes 47 be of such thinness that the heat conductivity through their walls will be as great as possible. Instead of employing tubes, it may be desirable to coat or line the inside of the uranium tubes 40 by spraying or otherwise depositing the protecting material directly on the inner surfaces of the uranium tubes. I n any event, there should be a proper bond between the lining or coating and the uranium tubes throughout their lengths to-efect an efficient heat transfer between these two members.

As shown, the cooling passages through the reactor are confined to the uranium tubes 40. Under thesecircum stanees, it is essential that an eicient heat conductivity also be maintained between the graphite and the uranium tubesb`ec`ause some heat is generated in the graphite and must be conveyed tothe uranium and removed from the system bythe coolingv medium passing through the uranium. Since space is provided around each of the uranium tubes 40, an effective heat transfer between the graphite and the uranium tubes is effected by filling vthis space with helium or some other low neutron absorbing heat conducting medium.

Due to the high temperatures produced in the reactor due to the chain reaction, provision is made for expansion of vthe helium gas. As illustrated in Fig. 2, gas expansion chambers indicated at 1Z0, 120e, and 12025 are connected to the reactor by means of a pipe 121. This pipe is located in the region ofthevbottom of the reactor. A tank filling and evacuating pipe 121e extends to the surface of the earth.

4As shown in Fig. 3, a horizontally disposed combination shield and water tank assembly is generally indicated at 48 and extends entirely over the top of the pile and is supported on the foundation 39. This assembly is made up of a lower enclosed tank 49 disposed horizontally across the top of the pile and serving as a shield, and an upper water inlet tank or header 50 coextensive with the lower tank. A gas seal is effected between the Water tank assembly 48 and the wall 39 by means of a sealing member indicated at 51 in Fig. 3. This sealing membei'v 51 is fastened to the assembly 48 and extends downwardly along the outer surface of the foundation 39 to a position below the ground level indicated at 37. The entire reactor is gas sealed so as to retain all gases inside the reactor.

The upper water tank 50 is divided into a plurality of tank segments providing an outer ring of medium pressurewater tanks 52 (Fig. 4) surrounding an inner core of high pressure water tanks 53. The ring header 25 completely surrounds the outer ring of medium pressure tanks 52, and passages 54 lead from this ring header to the medium pressure tanks 52. Une tank 55a in the outer ring of tanksserves as a header for the high pressure tanks 53 so that cooling water under relatively high pressure entering through the pipe 55 empties into the header tank 55a which, in turn, communicates with one of the high pressure tanks 53 through the passage 56. Each of the high pressure tanks 53 communicates with its next adjacent high pressure tank through a suitable passage 57.

The central group of cooling tubes 47, which are fed by water from the high pressure tanks 53 (Fig. 4), carry water under relatively high pressure while those tubes delivering water from the outer rings of tanks 52 carry water under a lower pressure. The reason for this pressure dilferential is -that more heat is generatedv at the center of the pilefthan is produced toward lthe sides. The rate of flow of water through the central high pressure tubes is about 16 meters per second, whereas the rate offlow through the medium pressure tubes is somewhat less than this.

Referring again -to Fig. 3, the graphite 41 and the uranium tubes 40 terminate at their upper extremities at about ground level. The water tubes 47, however, vproject upwardly above the uranium, pass through the shield 49 (see Fig. 6), and project into the upper water tank or header 50. In the space betweenthe top of the graphite 41 and the shield 49, the tubes 47 are coiled, as shown at 47a to provide Vfor expansion and contraction due to heat generated in the reactor.

The portion of each tube projecting through the shield 49 is surrounded by an iron sleeve 58 (Fig. 6,) which extends throughout the entire height of the shield and serves as a plug to absorb the neutrons and gamma ray radiations which otherwise would pass through the opening in the shield. Concentri-cally surrounding each plug 58 is a tube 59 sealed. at the top and bottom to the upper and lower walls, respectively, of the shield 49. There is sufficient clearance between the plug 58 and the tube 59 to permit free vertical movement of the plug relative to the tube.

At the top of each plug 58 (see Figs. 3 and 6), is a horizontally projecting peripheral flange 60 overlapping the upper wall of the shield 49. A suitable resilient ring gasket 61 is disposed between the underneath face of the flange 60 of the plug 58 and the upper wall of the shield 49.

In the space between the graphite 41 and the shield 49 are suitable beams 62 (Fig. 6) resting on the concrete foundation 20 (Fig. 3) and supporting the water tank and shield assembly 48.

The water tubes 47 communicate at the top with the interior of one of the water tank or header segments 52, 53, or 55a (depending upon the location of the tubes), so that water flows from the upper tank 50 through the water tubes 47 and discharges into the lower outlet-tank 43'from which it-runs by gravity into sump 26a.

The shield 49 is filled with iron shot 49a and water 49b, the combination of which provides a satisfactory neutron and gamma ray absorber so as to reduce the escape of these harmful radiations from the reactor to safe biological values for personnel on top of the pile. For a plant having an output of about 100,000 kilowatts, a satisfactory thickness for the shield 49 is 60 cm.

The water in the shield 49 must be cooled. Forfthis purpose, a separate cooling system may be employed. Referring to Fig. l Athe water is pumped out of the shield through a pipe 63 by a pump 64, and is passed through a suitable heat exchanger or cooler 65, and then is returned to the shield through a pipe 66. A cooling medium is passed in heat exchange relationship with the water flowing through the cooler 65.

Referring to Figs. 6 and 7, one satisfactory means for sealing the bottom of the uranium tubes 40 against the floor 42 is shown. Cooperating peripheral beads and grooves 67 and 68 respectively, together with a sealing ring 69, form this seal. The sealing ring 69 may be a lead ring. The weight of the uranium rod is supported ou the oor 42, the upper gasket 61 being sufficiently resiliend to permit the rod to rest on the floor.

In this arrangement, the seal is effected by the weight of the uranium rod or tube 40 which is forced by gravity onto the 'bead 67. Water vapor arising from the outlet tank 43 is thereby prevented from passing upwardly around the tubes 47 to corne in contact with the uranium tubes 40 or the graphite.

Another-arrangement for the bottom of the vuranium tubes is illustrated in Fig. 9. The same seal is used between the discharge header 43 andthe uranium, and a false door 7.2 is placed between the supporting floor 42 and the bottom of the discharge header 43. Between the false floor and the mainy supportingoor 42, a space 73 is provided forcirculation of helium gas by any convenient pipingy from separate lhelium tanks" (not shown) which will carry away any water vapor penetrating upwardly from the outlet tank 43. The tube 47 extends 

1. A COOLED NEUTRON CHAIN REACTING SYSTEM COMPRISING UPPER AND LOWER COOLANT TANKS IN VERTICALLY SPACED RELATIONSHIP, A LATTICE STRUCTURE DISPOSED IN THE SPACE BETWEEN THE TANKS AND COMPRISING ELONGATED URANIUM BODIES VERTICALLY DISPOSED IN AN EFFICIENT NEUTRON SLOWING AGENT, A NEUTRON AND GAMMA RAY ABSORBING SHIELD BETWEEN THE TOP OF THE LATTICE AND THE UPPER COOLANT TANK, MEANS PROVIDING ACCESS TO THE URANIUM BODIES FROM ABOVE THE SHIELD. A FLUID CIRCULATING TUBE EXTENDING THROUGH EACH URANIUM BODY AND COMMUNICATING WITH THE UPPER AND LOWER TANKS, THE TUBE SERVING TO CONVEY THE COOLANT THROUGH THE URANIUM BODY AND TO PROTECT THE URANIUM FROM ANY CORRO- 